This invention relates to a method of manufacturing the peripheral wall of a tank for an oil-immersed electric apparatus.
A tank for an oil-immersed electric apparatus is generally so constructed as to assure effective heat release from the surface of said tank. The known method of elevating heat release from such tank includes the process of fitting heat-releasing fins to the peripheral wall of the tank; the process of fitting heat-releasing pipes to the peripheral wall of the tank which are designed to communicate with the interior of the tank on the upper and lower sides; the process of welding the peripheral seam of two superposed panels to provide a hollow panel element, assembling a plurality of panel elements thus constructed, providing the top and bottom of said assembly with headers communicating with the hollow panel elements and attaching the final constructed unit to the peripheral wall of the tank; and constructing the peripheral wall of the tank by bending the thin steel sheet into hollow heat-releasing fins communicating with the interior of the tank thereby to enlarge the area of heat release. In recent years, the last mentioned process is chiefly accepted.
FIG. 1 shows the conventional tank peripheral wall constructed by bending the thin steel sheet into the above-mentioned hollow heat-releasing fins. The tank peripheral wall 1 is constructed by the steps of bending a thin band shaped steel plate 2 by means of a press at a prescribed distance thereby to provide a corrugated plate consisting of a plurality of closely arranged fins 1a having a cross section indicated in FIG. 2, welding together the separate walls of both upper and lower ends of the fins 1a as illustrated in FIG. 3, and welding a reinforcement member 3 to each fin 1a.
FIG. 4 shows a tank for an oil-immersed electric apparatus, said tank comprising the above-mentioned peripheral wall 1. This tank is constructed by welding the upper and lower edges of the tank peripheral wall 1 respectively to the upper frame 4 of the tank and its lower frame 5 concurrently used as the tank bottom.
The conventional process of fitting an insulation oil in the tank of an oil-immersed electric apparatus, comprises previously drying the interior of the tank to remove moisture, in order to assure the insulating property of said insulation oil, and carrying out said filling in vacuum. The vacuum filling of the insulation oil in undertaken either by holding the tank in a vacuum vessel and filling the insulation oil in the tank in vacuum or by evacuating the tank in the atmosphere and then filling the insulation oil in said tank. However, large capacity vacuum vessel must be provided in order to adopt the former process in filling the insulation oil in a large tank such as that used for a large capacity transformer. Therefore, the latter process is widely accepted in filling the insulation oil in such large capacity tank. When, in the case of the aforesaid tank constructed by bending its peripheral wall to provide fins, the filling of the insulation oil in the tank is carried out by the latter method, then both thin lateral walls of each hollow fin 1a are concavely deformed when the tank is evacuates due to the excessively great difference between atmospheric pressures prevailing inside and outside of said hollow fin 1a, thus eventually resulting in the crush of the fin 1a. Further, with an oil immersed electric apparatus, the internal pressure of the tank increases due to the expansion of the insulation oil whose temperature rises with the heating of the electric apparatus. Since the aforementioned tank has a thin peripheral wall, both lateral walls of the fin 1a cannot withstand the increased internal pressure of the tank, and consequently are convexly deformed, resulting in the possibility that the internal area of the tank grown large to give rise to a drop in the oil level.
Therefore, with respect to a tank whose fins are constructed by bending the peripheral wall of said tank, it is necessary to reinforce the fins by proper means. Throughout FIGS. 1 to 4, reference numeral 6 denotes reinforcement depressions formed in both lateral walls of the fin 1a to increase its mechanical strength. The depressions 6 are chosen to have a round shape. After the peripheral wall of the tank is bent to provide fins 1a, the depressions 6 are formed by pressing. The mutually facing depressions 6 on both sides of the fins 1a are made to abut against each other as shown in FIGS. 2 and 3, thereby to support both sides of the fins 1a from the inside and prevent their concave deformation. Further, the abutting sections of the depressions 6 are spot welded to join both sides of the fins 1a, thereby preventing their convex deformation.
With an oil-immersed electric apparatus tank whose hollow heat-releasing fins are constructed by bending the peripheral wall of the tank, it is necessary to cause the fins 1a to extend outward as much as possible in order to enlarge a heat-releasing area and elevate the heat-releasing effect. Hitherto, however, the fins 1a have been formed by bending the peripheral wall of the tank, failing to let the fins 1a extend as much outward as desired and consequently presenting difficultes in elevating the heat-releasing effect. The fin-forming method includes, for example, a folding process (West German disclosed patent application 2,620,419) in addition to the pressing process. Said West German folding process comprises clamping a band shaped steel plate at two points and narrowing the clamped spot by knocking up the intermediate portion of said clamped spot. This folding process can indeed theoretically enable the fins to extend noticeably outward, but in still accompanied with the drawbacks that it is necessary to provide an enlarged knocking-up device and let the clamping device have a long movement stroke, eventually causing the fin-forming apparatus to have an undesirably large scale, and it is practically impossible to let the fins extend as much outward as desired.
Further, the conventional fin-forming process comprises fabricating reinforcement depressions 6 after bending the peripheral wall of the tank to provide fins, and joining both lateral walls of the respective fins by spot welding the abutting portions of the reinforcement depressions 6. In this case, the pressing fabrication of the reinforcement depressions 6 and the spot welding of the abutting portions thereof have to be carried out in a space defined between the adjacent fins 1a, consuming a great deal of time. Hitherto, therefore, it has been impossible to efficiently manufacture the finned peripheral wall of a tank for an oil-immersed electric apparatus.